A2.1 Origins of cells

Conditions on early earth

1. Lack of atmosphere (4.5 billion years ago) which became a thick water vapour atmosphere (4 billion years ago). Thus, there was frequent lightning

2. Lack of oxygen results in no ozone layer and nothing to block UV light penetration, causing higher temperatures

3. The presence of gaseous compounds e.g. CO2, CH4, H2S that allowed penetration of UV light. CO2 and CH4 also retains heat. These factors contributed to high surface temperatures.

Why are viruses non-living?

Viruses do not carry out functions of life on their own. They take over machinery of infected living cells to carry out activities necessary for their reproduction

What are the functions of life?

Metabolism → convert energy from one form to another

Growth → may be limited but always present

Reproduction → passing on heriditary molecules to offspring

Response → adapt to the environment

Homeostasis → maintenance of a constant internal environment

Nutrition → breaking chemical bonds in compounds to provide energy for the organism

Excretion → release of unusable, toxic, or harmful chemical compounds from the organism

What is cell theory

• All organisms are composed of one or more cells

  • Proven by microscopes

• Cells are the smallest unit of life

  • No one has found a living entity that is not at least one cell large

• All cells come from pre-existing cells

  • Proven by experiments e.g. Pasteur experiment

What is the pasteur experiment

What are the requirements for the evolution of cells

Cells can currently only be producd by the division of pre-existing cells

• Catalysis

  • Synthesis of small carbon compounds from abiotic molecules (Miller-Urey experiment)

• Self-assembly

  • Small organic molecules joining to form polymers, which is accelerated with the presence of enzymes

  • Hot sand/rock/clay can also produce polymers as water from compounds is vapourised to form molecule chains.

• Emergence of compartmentalisation

  • Polymers contained by membranes to form protective homeostatic environment around polymers, separate from surroundings

  • Compartmentalisation of the cell

• Self-replication of molecules

  • Development of self-replicating molecules so that inheritance and control can occur.

What is the function of the parts of the miller urey experiment

1. Apparatus charged with simple compounds like CH4, NH3, H2 to represent early earh atmosphere

2. Heat to produce water vapour which goes to chamber containing inorganic compounds

3. Electrodes to simulate lightning

4. Cold water as condenser to collect gaseous compounds

5. Collecting device to collect sample

Evaluate the Miller-Urey experiment

Strengths

• presence of simple organic molecules in collecting device included hydrocarbons and amino acids

• Reproducible by others

• Results suggest life originated in primordial soup, a water-based sea of simple organic molecules

Weaknesses

• No one has definite knowledge about conditions on early earth

• Gases used may not be present → current gases in the experiment create a reducing environment favourable for the development of organic compounds. If gases were from volcanic mantle, the atmosphere would be non-reducing as the gases would be water vapour, CO2, SO2, small amounts of CO and H2S, which do not provide adequate hydrogen

• Proteins break down in water to produce individual amino acids, and amino acids have not been observed to form more complex structures in water.

Conclusion

The model does not provide reliable evidence of the first steps in the evolution of life due to its unprovable assumptions.

What are the alternative hypotheses to the miller urey experiment

Introduction of carbon by comets and meteroites that struck earth

Formation of atmosphere by volcanic fumes

Hydrothermal vents

What is the function of membranes

Barrier between inside of the cell and the surrounding environment

Regulation and maintenance of activities within the cell

Describe the formation of membranes

Possible due to the presence of fatty acids on early earth

• Fatty acids display a polarity with hydrophilic and hydrophobic properties

• When large numbers of fatty acids are placed in water, they tend to organise themselves into vesicles

• Vesicles produced in the lab can grow, engulf other organic molecules, and replicate themselves

What are the properties of RNA that make it the presumed first genetic material

1. RNA can assemble spontaneously from nucleotides, which are simpler organic molecules. This enables the appearance of RNA in early cell formation. (RNA is simpler than DNA)

2. RNA can form copies of itself and act as a type of genetic material

3. Varying types of nucleotides are capable of combining to form a larger molecule, which can carry code allowing inheritance of specific characteristics

4. RNA demonstrates the ability to control chemical reactions and act in an enzymatic role. Ribozymes in the ribosome are still used to catalyse peptide bond formation in protein synthesis.

What is the evidence for a last universal common ancestor

1. Universal genetic code carried by DNA and shared by all cells

2. Over 300 genes/sections of DNA common to all cells

3. Same building blocks of DNA/RNA in cells

4. Common molecular processes within all cells, including the replication of DNA molecules and transport of proteins

5. Similar transport mechanisms for cellular material within cells and in and out of cells

Is the last universal comon ancestor the first life on earth

no

What are fossils

Shells, bones, teeth

Remains of trails, footprints

Impressions of soft body parts

What are some dating techniques

Absolute dating

• Radiometric dating, based on half-life of fossils

• Fossils accumulate radioactive isotopes of certain elements at constant rates when they were alive

• Comparing amount of isotope in a fossil with the amount when it was alive shows age of fossil

Relative dating

• Uses sediment layers of earth and index fossils

• Sediments form layers in sequence, with the oldest at the bottom

• Index fossils are fossils of the same age at strata in different parts of the world

• It is assumed that all fossils in that strata should be the same age

• Geological processes may disturb the original sequence of layers which may provide inaccurate fossil ages.

Formation of communities around hydrothermal vents

Cracks in sealed crust causes water to rise, picking up minerals

Organisms form communities around the vents

• red-and-white tube worms that absorb the minerals and transfer them to symbiotic bacteria, which make food from the minerals that nourish the tube worm

This shows that lack of sunlight at bottom of the ocean does not result in no life being present, giving credibility to the formation of early life around hydrothermal vents

Evidence of LUCA at hydrothermal vents

1. Oldest fossilised traces/precipitates are there

2. Commonality of genetic sequences near the vents indicate a likely single ancestor

3. Presence of mineral-rich environment with both acidic and basic fluids required for chemical reactions

4. Presence of both H2 and CO2 to form the reducing environment essential for carbon compound formation